For the 2001 Ford Mustang, both the low-beam and high-beam headlights use a 9007 bulb. The fog lights use an 899 bulb, providing a separate fitment from the main headlamp assembly.
Other useful exterior fitments include 3157A for the front turn signals, parking lights, and front side markers. Rear lighting uses 3157R for the brake and tail lights. The rear turn signal varies by configuration: use 3157 on the Base model or 3156 on the SVT Cobra. Reverse lights use 3156, while the center stop light uses 906.
License plate lighting may use either 168 or 917, so compare the existing bulb before ordering. Interior applications include 578 for the dome and reading lights, 906 for the engine and trunk lights, and 194 or 7441 for the glove box, map, and auto-related locations. Courtesy and instrument lighting require comparison with the original bulb: courtesy uses 2721 or 212-2, while instrument lighting may use PC194, 161, or PC37.